X-ray Computed Tomography (CT) is a medical imaging tool providing cross-sectional images or images of “slices” of a human body. A patient is placed between an X-ray source and an array of detectors for measuring the amount of radiation passing through the patient's body. During the data acquisition process, the source and the detectors are rotated around the patient acquiring a large number of X-ray projection data. The X-ray projection data are then processed using a tomographic image reconstruction process in order to produce an image of a cross-section of the patient's body. For the data acquisition and the image reconstruction process it is assumed that the object—patient's organs—being scanned is stationary. This assumption is violated when there is any motion such as cardiac motion, blood flow, respiratory motion, or patient restlessness during the relatively long period of data acquisition, which is between 0.5 and 2.5 seconds for third generation X-ray CT scanners. The violation of this assumption causes motion artifacts, which appear as blurring, doubling and/or streaking in the reconstructed image. As is evident, such artifacts substantially impede diagnosis or even lead to erroneous diagnosis.
Several techniques for removing these artifacts have been disclosed in the prior art. For example, Crawford, C. R., King, K. F., Ritchie, C. J., and Godwin, J. D.: “Respiratory Compensation in Projection Imaging using a Magnification and Displacement Model”, IEEE TMI, 15(3), pp. 327–332, 1996, teach a linear model for the motion, while Ritchie, C. J., Hsieh, J., Gard, M. F., Godwin, J. D., Kim., Y, and Crawford, C. R.: “Predictive Respiratory Gating. A New Method to Reduce Motion Artifacts in CT Scans”, Radiology, 190, pp. 847–852, 1994, model the motion as periodic and take projection data at a particular point in the motion cycle, producing a stroboscope-like effect. However, the organ motion is much more complex and these techniques have a severely limited ability for correcting organ motion. In a more general technique disclosed by Chiu, Y. H., and Yau, S. F.: “Tomographic Reconstruction of Time-Varying Object from Linear Time-Sequential Sampled Projections”, Proc. IEEE Conf. ASSP, Adelaide, 1, pp. V309–V312, 1994, motion effects are iteratively suppressed from the projection data. This process requires proper initialization in order to achieve an acceptable convergence period. Another technique based on retrospective gating is taught by Morehouse, C. C., Brody, W. R., Guthaner, D. F., Breiman, R. S., and Harell, G. S.: “Gated Cardiac Computed Tomography with a Motion Phantom”, Radiol., 134(1), pp. 213–217, 1980, and employs an ECG signal to identify the diastole phase of a patient's cardiac cycle—i.e. when the heart is least active—and use the projection data collected during the diastole phase to reconstruct the tomographic image.
Although the correction methods disclosed in the above cited references reduce the adverse effects of motion to some extent, none of these methods substantially removes the motion artifacts from the reconstructed tomographic images.